1/x（x+1）+ 1/(x+1)(x+2) + 1/(x+2)(x+3)+...+1/(x+8)(x+9)
=[1/x-1/(x+1)]+[1/(x+1)-1/(x+2)]+[1/(x+2)-1/(x+3)]+...+[1/(x+8)-1/(x+9)]
=1/x-1/(x+9)
=9/[x(x+9)]